Method of communication with improved acknowledgment of reception

ABSTRACT

A method for communicating between a transmitting unit and a receiving unit. A messages formed by elementary messages is transmitted from the transmitting unit to the receiving unit, and at least one reception bit is transmitted from the receiving unit to the transmitting unit. The reception bit (or bits) allows the transmitting unit to determine the elementary message that is to be transmitted next. In a preferred method, at least two reception bits are transmitted from the receiving unit and the values of the reception bits indicate the elementary message that is to be transmitted next by the transmitting unit. The present invention also provides a receiving device for receiving messages from a transmitting device. The receiving device includes an interface for receiving a transmitted message from the transmitting device, means for analyzing a received elementary message to determine if it was properly received, and a transmitter for transmitting at least one reception bit to the transmitting device. The reception bit (or bits) indicates the elementary message that is to be transmitted next by the transmitting unit. In one preferred embodiment, the transmitter transmits at least two reception bits whose values indicate the elementary message that is to be transmitted next by the transmitting unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of U.S. Application Serial No. 09/234,131,filed Jan. 15, 1999, now U.S. Pat. No. ______. The entire disclosure ofprior application Ser. No. 09/234,131 is herein incorporated byreference.

[0002] Additionally, this application is based upon and claims priorityfrom prior French Patent Application No. 98-00450, filed Jan. 16, 1998,the entire disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to digital transmission, and morespecifically to an efficient method for communicating between master andslave units by using a protocol having improved acknowledgment duringmessage reception.

[0005] 2. Description of Related Art

[0006] Digital transmissions can be made by means of different media.The transmission channels may indeed be either wire links, opticalfibers, or a volume of air between two antennas. FIG. 1 shows an exampleof communication using RF links. As shown, a master unit 1 uses atransceiver antenna 11 to exchange messages with a slave unit 2 having atransceiver antenna 22. The transmission channel is formed by the volumeof air between the two antennas 11 and 22. Each of the master unit andthe slave unit can transmit messages to the other unit and receivemessages from the other unit. The master unit is distinguished from theslave unit in that the master unit takes the initiative in thecommunication. For example, the master unit can be the centralprocessing unit of a computer, and the slave unit can be one of itsperipherals such as a printer that is remote-controlled by means of anytransmission channel.

[0007] There are conventional methods of communication that consist oftransmitting messages that include a useful information word and one ormore service bits. The transmission is carried out serially according toa specified communications protocol. A protocol of this kind specifiesthe format and the syntax of the messages that are transmitted by themaster unit to the slave unit or vice versa. The transmission is donesynchronously, and is sequenced at a specified rate with the unitsincluding conventional circuitry to lock in to the rate the same clockor two synchronous or dia-synchronous clocks.

[0008]FIG. 2 shows an example of a conventional elementary messageformat. The elementary message begins with a starting bit START whosefunction is to synchronize the clock of the addressed unit with thereceived message. Then, the message includes an encoded usefulinformation word INFO (e.g., encoded in eight bits or one byte). Thisword may be an instruction word whose value indicates the nature of acommand to be carried out by the addressed unit. For example a read orwrite command. Alternatively, it could be an address word whose valueindicates the address (or a part of the address) of a memory location ofthe addressed unit at which a data element is to be read or written.Additionally, the word can be a data word whose value indicates thevalue of a data element processed by the addressed unit.

[0009] The elementary message also includes a check bit CHECK such as aparity check bit. The value of the parity check bit CHECK is fixed atthe logic value 1 or 0, and is determined in such a way that the sum ofthe values of the bits of the useful information INFO gives an evenvalue or an odd value depending on the chosen type of parity. The roleof the parity check bit CHECK is to enable the addressed unit to detectany transmission errors. In such a case, the addressed unit can requesta re-transmission of the message.

[0010] The conventional message ends with an end-of-transmission bitSTOP that is used solely to indicate the end of the message. Followingthe end-of-transmission bit STOP, each protocol generally provides for anumber of elementary temporal units during which the sending unit nolonger sends any bit on the transmission channel. Thus, the transmissionchannel is left free so that the addressed unit can send a bit ACK toacknowledge the communication. The logic value of the acknowledgment bitindicates whether the message has been accurately received. Thecircuitry conventionally used to determine whether the message has beenaccurately received is the check performed by means of the parity checkbit CHECK. In FIG. 2, four such elementary temporal units follow the bitSTOP, with one of these temporal units being occupied by the receptionacknowledgment bit ACK.

[0011] There presently exists a very large quantity of differentcommunication protocols for electronic systems. Each protocol is suitedto the specific constraints of a specific application (such constraintsincluding the size of the words to be transmitted, coherence checks, theneed to securitize communications against passive or active interventionby ill-intentioned individuals, constraints related to acknowledgmentmode, and maximum duration of transmission in relation to the bit rate).Within one system, the master unit and the slave unit obviously conformthe same communications protocol so that they can communicate with eachother intelligibly.

[0012] In general, the information on the communications protocolsdeveloped by electronics systems manufacturers is widely disseminated inorder to enable other manufacturers to incorporate these systems intomore complex assemblies or develop new industrial and/or commercialapplications. Thus, with respect to the size of the communications inwhich such protocols are used, the integrity of the informationtransmitted and the efficiency of the transmission has to be theoptimum. In other words, it is necessary for the transmission to beaffected by a minimum of errors. Furthermore, there are applications inwhich the integrity of the information exchanged and the efficiency ofcommunication are major constraints in the specifications.Conventionally, quality control of the transmission of the binarysignals lies essentially in the analysis and interpretation of the valueof the parity check bit CHECK described above.

[0013]FIGS. 3A and 3B show two possible cases to illustrate thedetermining of the parity bit CHECK. In these examples, the messageformat of FIG. 2 is used. In the figures, no special attention has beenpaid to the values of the starting bit START and end-of-transmission bitSTOP. All that has been taken into account is the values of the bits ofthe useful information word INFO, the parity check bit CHECK, and theacknowledgment bit ACK. FIG. 3A shows the case of a useful informationword INFO1 whose transmission has not been erroneous. The logic value 1is illustratively given to the first seven bits B1, B2, B3, B4, B5, B6,and B7 of the useful information word INFO1, and the last bit B8 of theuseful information word INFO1 is set at the logic value 0.

[0014] If an “even” type of parity is used, the parity check bit CHECKwill then take the logic value 1. That is, the sum of the values of theeight bits of the useful information word INFO1 added to the value ofthe parity bit CHECK gives the number 8 in conformity with even parity.In the case of FIG. 3A, the transmission of the useful information wordINFO1 is not affected by errors so the useful information word INFO2that results from the transmission of the useful information word INFO1consequently has the same binary values for each of the bits. Ingeneral, a circuit formed by elementary logic gates is used to ascertainthat the result of the transmission of the useful word is in conformitywith the expected result with respect to the value of the receivedparity check bit and the type of parity adopted. Thus, as shown in FIG.3A, the acknowledgment bit has the value that indicates accuratereception. (In FIGS. 3A and 3B, accurate acknowledgment illustrativelycorresponds to a value 1 of the acknowledgment bit ACK.)

[0015]FIG. 3B shows a case with the same useful information word INFO1,but for a transmission error. A useful information word INFO3 isreceived in the reception unit following the transmission of the usefulinformation word INFO1. In FIG. 3B, the eighth bit B8 of the receiveduseful information word INFO3 is different from the eighth bit of thetransmitted useful information word INFO1. Thus, the transmission hasbeen erroneous and the useful information word INFO3 no longer matcheswith the parity check bit CHECK. The same result would occur if three,five, or seven of the eight bits constituting the received usefulinformation word INFO3 had been different from the bits of thetransmitted useful information word INFO1. Indeed, in all these cases,the sum of the eight bits of the useful information word INFO3 added tothe value of the parity check bit CHECK gives an odd number while thetype of parity is even parity. Consequently, in FIG. 3B, theacknowledgment bit ACK has the value 0.

[0016] The transmission of a message includes three or four phases ofreception and transmission depending on whether the operation is in readmode or in write mode. The order of these phases is important. Forexample, for a write operation, a sequence for the transmission of amessage can be subdivided for example into four phases of transmissionthat correspond, for each phase, to the transmission of an elementarymessage containing a useful information word of a different type. Inwrite mode, the first transmission phase is the transmission of a codepertaining to an instruction to be performed, and the secondtransmission phase is the transmission of the least significant bits ofthe memory address at which it is sought to write a data element. Thethird transmission phase is a transmission of the most significant bitsof the same memory address, and the fourth transmission phase is atransmission of the bits constituting the data element to be written atthe memory address.

[0017] For a read operation, a sequence for the transmission of amessage can be subdivided into three phases of transmission thatcorrespond, for each phase, to the transmission of an elementary messagecontaining a useful information word of a different type. The threetransmission phases are the three first phases described above for awrite operation. Conventionally, when an error is detected by a testrelating to the parity check bit, the transmission sequence is resumedat the first transmission phase of the message, regardless of thecurrent transmission phase. Thus, even if earlier phases of thetransmission have been accurately acknowledged, such transmission phasesmust be undertaken again as part of the entire transmission process.This lowers the efficiency of the transmission.

SUMMARY OF THE INVENTION

[0018] In view of these drawbacks, it is an object of the presentinvention to remove the above-mentioned drawbacks and to provide anacknowledgment of reception that enables a retransmission of the currentphase of the transmission sequence. A counter is provided that can beused to count the number of retransmissions of one and the sametransmission phase. At the end of a predetermined number ofre-transmissions of the same phase of the transmission sequence, thecounter is reset and the transmission sequence is again performedstarting from the first transmission phase.

[0019] One embodiment of the present invention provides a method forcommunicating between a transmitting unit and a receiving unit.According to the method, a messages formed by elementary messages istransmitted from the transmitting unit to the receiving unit, and atleast one reception bit is transmitted from the receiving unit to thetransmitting unit. The reception bit (or bits) allows the transmittingunit to determine the elementary message that is to be transmitted next.Thus, an improvement is introduced into the protocol to determine thefollowing transmission phase in a transmission sequence. In a preferredmethod, at least two reception bits are transmitted from the receivingunit and the values of the reception bits indicate the elementarymessage that is to be transmitted next by the transmitting unit.

[0020] Another embodiment of the present invention provides a receivingdevice for receiving messages from a transmitting device. The receivingdevice includes an interface for receiving a transmitted message fromthe transmitting device, means for analyzing a received elementarymessage to determine if it was properly received, and a transmitter fortransmitting at least one reception bit to the transmitting device. Thereception bit (or bits) indicates the elementary message that is to betransmitted next by the transmitting unit. In one preferred embodiment,the transmitter transmits at least two reception bits whose valuesindicate the elementary message that is to be transmitted next by thetransmitting unit.

[0021] Other objects, features, and advantages of the present inventionwill become apparent from the following detailed description. It shouldbe understood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the presentinvention, are given by way of illustration only and variousmodifications may naturally be performed without deviating from thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a block diagram showing a master unit and a slave unitthat can exchange messages;

[0023]FIG. 2 is a diagram showing a conventional format for anelementary message;

[0024]FIGS. 3A and 3B are diagrams showing exemplary messages thatillustrate the function of a parity check bit;

[0025]FIG. 4 is a diagram showing a format for an elementary messageaccording to one embodiment of the present invention;

[0026]FIG. 5 is a flow chart showing a process for sending a messageaccording to a preferred embodiment of the present invention;

[0027]FIG. 6 is a flow chart showing a process for receiving a messageaccording to the preferred embodiment of the present invention; and

[0028]FIG. 7 is a block diagram showing one embodiment of a device forimplementing the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] Preferred embodiments of the present invention will be describedin detail hereinbelow with reference to the attached drawings.

[0030] As described above with respect to FIG. 2, after transmission ofa stop bit, the conventional protocols generally provide for a number ofelementary temporal units during which a transmission unit no longersends out any bits on the transmission channel. The transmission channelis thus left free so that the addressed unit can send a communicationsacknowledgment bit. Depending on the logic value of the acknowledgmentbit, the transmitted elementary message is considered as having beenaccurately or erroneously received. In the former case, the operationproceeds to the next phase of the message sending sequence. On the otherhand, in the latter case, the entire initial sequence of the messagemust be repeated under the conventional protocols.

[0031] The present invention presents an improvement in the role of theacknowledgment bit that cause the transmission unit either to transmitthe next phase of the message transmission sequence, to re-transmit thelast transmitted phase of the message transmission sequence, or tore-transmit the entire message transmission sequence. In a firstembodiment of the present inventions, the value of the acknowledgmentbit ACK and its position in the elementary temporal units left free bythe transmission unit is taken into account. The acknowledgment bit isthen the carrier of a piece of information whose content is greater thana binary piece of information.

[0032] The elementary message format according to another embodiment ofthe present invention is shown in FIG. 4. This elementary message formatcorresponds to the conventional format shown in FIG. 2 except that eachmessage has two reception acknowledgment bits ACK1 and ACK2. The twobits ACK1 and ACK2 form a reception acknowledgment word MACK and eachelementary message is therefore provided with two acknowledgment bitsforming an acknowledgment word. Consequently, the acknowledgment wordmay convey an information element whose content is greater than a simplebinary piece of information. These messages make it possible to informthe transmission unit about the phase of the message transmissionsequence that has to be sent.

[0033]FIG. 5 shows a flow chart for sending a message according to apreferred embodiment of the present invention. In FIG. 5, a letter kdesignates an index that can assume a value from 0 to 4 to signify thecurrent phase of the transmission sequence, a letter S always determinesthe phase number in the transmission sequence, and the values VAL1,VAL2, and VAL3 designate the different values that can be assumed by theacknowledgment word MACK. In particular, the value VAL1 means that thereception has been properly acknowledged with respect to the paritycheck bit, the value VAL2 means that the reception has not been properlyacknowledged with respect to the parity check bit but that it is desiredto resume the transmission sequence at the current phase of thetransmission sequence (i.e., so the last elementary message can be sentagain), and the value VAL3 corresponds to the value assumed by theacknowledgment word when the reception has not been accuratelyacknowledged and when it is desired to resume transmission from thefirst phase of the message transmission sequence.

[0034] As shown, in a first step 101 after the beginning of the program,a test is performed to determine if the operation is in the first phaseof the transmission sequence. The value of k is then 0. If the answer isyes, then the useful information word is a control instruction. Then, atest 102 allows, through a decoding of the bits of the usefulinformation word, to determine whether the useful information wordcorresponds to a write instruction. If the answer is yes, then in a step103, the natural integer S assumes the value 4. If the answer is no(i.e., if the instruction is a read command), then in step 104, thenatural integer S assumes the value 3.

[0035] Once the number of phases of the transmission sequence has beendetermined, the message is effectively transmitted in step 105.Following step 105, step 106 consists of the reception of theacknowledgment word MACK. Then, in step 107 the value of the word MACK,is used to determine whether the last transmission has been accuratelyacknowledged by the reception unit. If the transmission has beenaccurately acknowledged by the reception unit, the index k isincremented by one unit in the step 108. Following step 108, in step109, a test is performed to determine whether the value of the index khas reached the value of the previously fixed integer S. If the responseis positive, the transmission sequence is completed. In step 110, theindex k is then reset to the value 0 and this is the end of the process.If the response in step 109 is negative, the program loops back to step101.

[0036] If the response in step 107 (i.e., in the test used to determinewhether the transmission has been accurately acknowledged) is negative,then another test is performed in step 111. This test compares theacknowledgment word MACK with the value VAL2. If these two values areequal, the program resumes at step 101. In such a case, the transmissionhas not been accurately acknowledged and the current phase of themessage transmission sequence is reiterated. Alternatively, if theresponse to this test is negative, the variable k is reset to the value0 in step 112 and the process is resumed at step 102. This correspondsto a reception that is not properly acknowledged and to a resumption ofthe transmission sequence at the first phase of the sequence.

[0037]FIG. 6 shows a flow chart for receiving a message according to apreferred embodiment of the present invention. In this figure, thevariables k′ and S′ are used as natural integers, whose roles areidentical to the roles of the variables k and S in FIG. 5. Additionally,a variable CMPT is used in the process of FIG. 6 to enable the countingof the number of successive transmissions of one and the same phase ofthe sequence for the transmission of a message. Further, a constant LIMis introduced and chosen arbitrarily to enable the number of successivetransmissions of one phase of a transmission sequence to be limited tothe value assigned to it.

[0038] In a first step 200 after the beginning of the program, themessage sent by the transmission unit is received. Next, in step 201 atest is performed to find out if the program is in the first phase ofthe reception sequence (i.e., if k′=0). If the response is yes, then theuseful information word is necessarily a control instruction. A test atstep 202 then makes it possible, through a decoding of the bits of theuseful information word, to determine whether the useful informationword corresponds to a write instruction. If the answer is yes, then thenatural integer S′ assumes the value 4 in step 203. If the answer is no,then the natural integer S′ assumes the value 3 in step 204.

[0039] At the end of step 204, or at the end of step 202 if the responseto the test at step 201 is negative, a parity test 205 is performed. Ifthe result of the parity test 205 proves to be right according to theprinciples explained above, then step 206 assigns the value VAL1 to theacknowledgment word MACK. Then, in step 207, the variable CMPT is resetto 1. Following step 207, in step 208 the index k′ is incremented by oneunit, and then a test is performed at step 209 to compare the integersk′ and S′. If the values are equal (i.e., if the reception sequence isin its last phase), then the index k′ is reset to 0 in step 210 and theright acknowledgment word, whose value is VAL1, is sent in step 211. Theend of the process is then reached.

[0040] If the parity test at step 205 has revealed a transmission error,the variable CMPT is incremented by one unit in a step 212. Followingstep 212 a test at step 213 compares the variable CMPT with the constantLIM. If the two values are different, in step 214 the value VAL2 isassigned to the acknowledgment word MACK. Thus, a re-transmission of thelast elementary message transmitted is needed. This acknowledgmentmessage is sent in a step 218. On the other hand, if the values of thevariables CMPT and LIM are equal, in step 215 the value VAL3 is assignedto the acknowledgment word MACK.

[0041] Following step 215, two steps 216 and 217 are used to reset thevariable CMPT to the value 1 and the variable k′ to the value 0. Thesending of the acknowledgment word MACK is then performed in step 218. Are-transmission of the entire message transmission sequence is thusrequired. Thus, in the event of a transmission that has not beenproperly acknowledged, the transmission of the last elementary messagetransmitted is requested a certain number of times before thetransmission is resumed at the first elementary message of the currentmessage. In any case, step 218 replaces the performance of a process atstep 200 that corresponds to the reception of a phase of thetransmission sequence.

[0042]FIG. 7 is a block diagram of one embodiment of a device forimplementing the present invention. A slave unit 2 includes a receptionunit 70 that receives an elementary message and circuitry 71 thatanalyzes the coherence of the elementary message received with respectto the value of the parity check bit. This circuitry means 71 can bebased on elementary logic gates. Furthermore, an assigning circuit 72assigns the appropriate reception bit ACK or the appropriate receptionword MACK (depending on the message format used). A counter 73 isincremented if a re-transmission is demanded, and can be reset by aresetting circuit 74 after a number of re-transmissions of the sameelementary message. Finally, the slave unit 2 includes a transmissionunit 75 that sends the reception unit ACK or the appropriate receptionword MACK.

[0043] While there has been illustrated and described what are presentlyconsidered to be the preferred embodiments of the present invention, itwill be understood by those skilled in the art that various othermodifications may be made, and equivalents may be substituted, withoutdeparting from the true scope of the present invention. Additionally,many modifications may be made to adapt a particular situation to theteachings of the present invention without departing from the centralinventive concept described herein. Furthermore, embodiments of thepresent invention may not include all of the features described above.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the invention include allembodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for communicating between a transmittingunit and a receiving unit over a synchronous serial link, thetransmitting unit being a master unit and the receiving unit being aslave unit, said method comprising the steps of: transmitting a messageformed by elementary messages from the transmitting master unit to thereceiving slave unit over the synchronous serial link, each of theelementary messages including a useful information word; establishing atime gap composed of a plurality of elementary temporal units after eachelementary message is transmitted by the transmitting master unit, thetransmitting master unit not transmitting over the synchronous seriallink during the time gap; and during the time gap, receiving anacknowledgment word from the receiving slave unit, the acknowledgmentword being received by the transmitting master unit via the synchronousserial link, wherein the acknowledgment word includes at least onereception bit, the at least one reception bit being selectivelypositioned at one of the elementary temporal units of the time gap, andthe transmitting master unit determines the elementary message that isto be transmitted next based at least partially on at which elementarytemporal unit of the time gap the received at least one reception bit ispositioned.
 2. The method as defined in claim 1, wherein theacknowledgment word consists of one reception bit, and the elementarytemporal unit of the time gap at which the reception bit is positionedindicates the elementary message that is to be transmitted next by thetransmitting unit.
 3. The method as defined in claim 1, wherein theacknowledgment word consists of one reception bit, and the value of thereception bit and the elementary temporal unit of the time gap at whichthe reception bit is positioned indicate the elementary message that isto be transmitted next by the transmitting unit.
 4. The method asdefined in claim 1, further comprising the step of: if theacknowledgment word indicates that the elementary message was notproperly received, re-transmitting the previous elementary message fromthe transmitting unit to the receiving unit.
 5. The method as defined inclaim 4, wherein the step of re-transmitting the previous elementarymessage is repeated up to a predetermined number of times.
 6. The methodas defined in claim 5, further comprising the step of: if the step ofre-transmitting the previous elementary message is repeated thepredetermined number of times and the transmitting unit determines thatthe elementary message was not properly received, re-transmitting thefirst elementary message of the message.
 7. The method as defined inclaim 1, wherein each of the elementary messages further includes aparity check bit.
 8. The method as defined in claim 7, furthercomprising the step of determining at least the value of at least one ofthe reception bits using at least the parity check bit.
 9. Amachine-readable medium encoded with a program for communicating betweena transmitting unit and a receiving unit over a synchronous serial link,the transmitting unit being a master unit and the receiving unit being aslave unit, said program containing instructions for performing thesteps of: transmitting a message formed by elementary messages from thetransmitting master unit to the receiving slave unit over thesynchronous serial link, each of the elementary messages including auseful information word; establishing a time gap composed of a pluralityof elementary temporal units after each elementary message istransmitted by the transmitting master unit, the transmitting masterunit not transmitting over the synchronous serial link during the timegap; and during the time gap, receiving an acknowledgment word from thereceiving slave unit, the acknowledgment word being received by thetransmitting master unit via the synchronous serial link, wherein theacknowledgment word includes at least one reception bit, the at leastone reception bit being selectively positioned at one of the elementarytemporal units of the time gap, and the transmitting master unitdetermines the elementary message that is to be transmitted next basedat least partially on at which elementary temporal unit of the time gapthe received at least one reception bit is positioned.
 10. Themachine-readable medium as defined in claim 9, wherein theacknowledgment word consists of one reception bit, and the elementarytemporal unit of the time gap at which the reception bit is positionedindicates the elementary message that is to be transmitted next by thetransmitting unit.
 11. The machine-readable medium as defined in claim9, wherein the acknowledgment word consists of one reception bit, andthe value of the reception bit and the elementary temporal unit of thetime gap at which the reception bit is positioned indicate theelementary message that is to be transmitted next by the transmittingunit.
 12. A receiving device for receiving messages from a transmittingdevice over a synchronous serial link, the receiving device being aslave device and the transmitting device being a master device, saidreceiving device comprising: an interface for receiving a transmittedmessage from the transmitting master device via the synchronous seriallink, the message being formed by elementary messages that each includeat least one check bit; means for analyzing a received elementarymessage to determine if the received elementary message was properlyreceived based on the value of the check bit; and a transmitter fortransmitting, during a time gap composed of a plurality of elementarytemporal units after an elementary message is transmitted by thetransmitting master device, an acknowledgment word to the transmittingmaster device over the synchronous serial link, wherein theacknowledgment word includes at least one reception bit, the at leastone reception bit being selectively positioned at one of the elementarytemporal units of the time gap, and the elementary temporal unit of thetime gap at which the at least one reception bit is positioned is afunction of the elementary message that is to be transmitted next by thetransmitting master device.
 13. The receiving device as defined in claim12, wherein the acknowledgment word consists of one reception bit, andthe elementary temporal unit of the time gap at which the reception bitis positioned indicates the elementary message that is to be transmittednext by the transmitting master device.
 14. The receiving device asdefined in claim 12, wherein the acknowledgment word consists of onereception bit, and the value of the reception bit and the elementarytemporal unit of the time gap at which the reception bit is positionedindicate the elementary message that is to be transmitted next by thetransmitting master device.
 15. A method for communicating between atransmitting unit and a receiving unit over a synchronous serial link,the transmitting unit being a master unit and the receiving unit being aslave unit, said method comprising the steps of: receiving a messagefrom the transmitting master unit at the receiving slave unit via thesynchronous serial link, the message being formed by elementary messagesthat each include a useful information word; during a time gap composedof a plurality of elementary temporal units after an elementary messageis transmitted by the transmitting master unit, transmitting anacknowledgment word from the receiving slave unit to the transmittingmaster unit over the synchronous serial link, wherein the acknowledgmentword includes at least one reception bit, the at least one reception bitbeing selectively positioned at one of the elementary temporal units ofthe time gap, and the elementary temporal unit of the time gap at whichthe at least one reception bit is positioned is a function of theelementary message that is to be transmitted next by the transmittingmaster unit.
 16. The method as defined in claim 15, wherein theacknowledgment word consists of one reception bit, and the elementarytemporal unit of the time gap at which the reception bit is positionedindicates the elementary message that is to be transmitted next by thetransmitting master unit.
 17. The method as defined in claim 15, whereinthe acknowledgment word consists of one reception bit, and the value ofthe reception bit and the elementary temporal unit of the time gap atwhich the reception bit is positioned indicate the elementary messagethat is to be transmitted next by the transmitting master unit.
 18. Themethod as defined in claim 15, wherein if the elementary message was notproperly received by the receiving slave unit, the acknowledgment wordindicates that the transmitting master unit should re-transmit theprevious elementary message to the receiving slave unit.
 19. The methodas defined in claim 18, if the re-transmitting of the previouselementary message has been repeated a predetermined number of times andthe elementary message was not properly received by the receiving slaveunit, the acknowledgment word indicates that the transmitting masterunit should re-transmit the first elementary message of the message. 20.A transmitting device for transmitting messages to a receiving deviceover a synchronous serial link, the transmitting device being a masterdevice and the receiving device being a slave device, said transmittingmaster device comprising: a transmitter for transmitting a message tothe receiving slave device over the synchronous serial link, the messagebeing formed by elementary messages that each include at least one checkbit; means for establishing a time gap composed of a plurality ofelementary temporal units after each elementary message is transmitted,the transmitter not transmitting over the synchronous serial link duringthe time gap; and an interface for receiving, during the time gap, anacknowledgment word from the receiving slave device via the synchronousserial link, wherein the acknowledgment word includes at least onereception bit, the at least one reception bit being selectivelypositioned at one of the elementary temporal units of the time gap, andthe transmitting master device determines the elementary message that isto be transmitted next based at least partially on at which elementarytemporal unit of the time gap the received at least one reception bit ispositioned.
 21. The transmitting device as defined in claim 20, whereinthe acknowledgment word consists of one reception bit, and theelementary temporal unit of the time gap at which the reception bit ispositioned indicates the elementary message that is to be transmittednext by the transmitting master device.
 22. The transmitting device asdefined in claim 20, wherein the acknowledgment word consists of onereception bit, and the value of the reception bit and the elementarytemporal unit of the time gap at which the reception bit is positionedindicate the elementary message that is to be transmitted next by thetransmitting master device.